Time resolved studies of intersubband relaxation in GaAs/AlGaAs quantum wells below the optical phonon energy using a free electron laser

The problem of intersubband relaxation in GaAs/GaAs quantum wells, where the energy separation of the two lowest subbands is smaller than the optical phonon energy, is considered. Time resolved pump-and-probe measurements are performed with a far-infrared free-electron laser on two multiquantum well samples with similar thicknesses (approximate to 300 Angstrom), but different doping and mobilities. The measured lifetimes are shorter than could be explained by acoustic phonon emission alone. Monte-Carlo calculations show the importance of electron-electron scattering for thermalization of the hot electron distribution function and subsequent optical phonon emission from the long thermal tail. (C) 1996 Academic Press Limite